The reason is mostly that sales wants to someone who walks in the door the first time to walk out. If someone wants to custom order a car the 10^180 possible combinations isn't that big a deal, but there is no way to have that many cars on every lot and so sales demands it.
Yes less different models does save a little but of cost, but it isn't that much overall.
Yes less different models does save a little but of cost, but it isn't that much overall.